Axial Wall Taper and Undercuts—Why the Crown Doesn't Seat

A crown that doesn't fully seat, or seats with resistance—this is most often a problem with axial wall taper or undercuts in the preparation.

Axial Wall Taper—Why 6–10°?

An ideal preparation has axial walls tapering occlusally at an angle of 6–10° (total occlusal convergence, TOC). This range ensures:

• Retention for conventional cement—the crown does not dislodge
• Resistance to rotation—the crown does not rotate under masticatory forces
• Absence of undercuts—the crown seats along its path of insertion

Too little taper (<5°): The walls are almost parallel. The crown geometrically cannot seat because the slightest irregularity blocks the path of insertion. The technician must either relax the fit (loose crown) or return the work.

Too much taper (>15°): The crown seats easily, but retention drops dramatically. With conventional cement—risk of the crown coming off when chewing gum or sticky food.

With adhesive cementation (e.g., e.max), the tolerance is slightly higher—up to 12°. But still: the more parallel the walls, the better the retention.

Undercuts—When They Are a Problem

An undercut is an area in the preparation that is wider in its depth than at its entrance. In other words—a part of the abutment that blocks the crown's path of insertion.

Undercuts most often occur:

• On the lingual wall of mandibular molars—the tongue makes drill control difficult
• In the interdental papilla area—the drill approaches at an angle
• When preparing tilted teeth—the tooth axis does not align with the path of insertion
• On implant abutments with a non-standard angle

Result: the crown does not seat fully. A gap remains between the crown and the abutment—risk of secondary caries, the cement does not seal the margin.

How the Lab Detects Undercuts

At deltalabs., every preparation scan is analyzed in CAD software for undercuts and taper:

• Undercut map—colored visualization of areas blocking the path of insertion
• TOC measurement—automatic measurement of taper on each wall
• Seating simulation—digital insertion of the crown onto the abutment with collision control

If undercuts are small (up to 0.3 mm)—we block them in the CAD design (add material to the internal surface of the crown). If larger—we contact the dentist before proceeding with the work.

What to Do When the Crown Doesn't Seat

In the clinic:

1. Check the contact point—too tight a contact with the adjacent tooth blocks the crown
2. Check the margin—cement material or soft tissue can block seating
3. Use occlusal spray (Occlude, Fit Checker)—it will show exactly where the crown is blocking

If the problem is an undercut on the abutment:

1. Gentle preparation correction (removal of the undercut) with a fine-grit diamond bur
2. New scan/impression and new crown—if the undercut is large
3. Change in path of insertion—possible for single crowns, impossible for bridges

Prevention—How to Avoid Taper-Related Problems

• Use an occlusal mirror to check taper from all sides
• When preparing, verify the path of insertion from all sides—not just occlusally
• For tilted teeth, plan the path of insertion before preparation
• For multi-unit bridges, establish a common path of insertion beforehand
• Intraoral scan after preparation allows immediate visualization of undercuts on the screen

1. Does a CAD/CAM crown fit better than a traditional one?

Yes. The fit accuracy of CNC-milled crowns is 20–50 µm, whereas the traditional method (wax + casting) yields 50–120 µm. Fewer undercuts are blocked because digital design compensates for them.

2. What if the lab reports undercuts in my preparation?

This is not a criticism—it is information that allows you to make a decision: correct the preparation, accept blocking in CAD, or change the path of insertion. At deltalabs., we always contact the clinician before proceeding with the work if undercuts exceed the norm.

3. Does adhesive cement solve the problem of poor taper?

Partially. Adhesive cement (e.g., for e.max) compensates for lower mechanical retention with chemical bonding. But too much taper (>15°) is a problem even for adhesive cement—lateral forces can dislodge the crown.